The present invention relates to the field of analytic chemistry, and especially to a mass spectrometer including a quadruple ion-trap type mass spectrometer unit.
A plasma ionizing mass spectrometer method, in which a sample is analyzed by being put in plasma generated at atmospheric pressure, and having the ionized sample introduced into the plasma in a vacuum, is well known as a high-sensitivity elemental analysis method. The general apparatus most generally used for this method is an inductively coupled plasma-mass spectrometer (hereafter abbreviated to ICP-MS). Plasma is generated by using a high frequency radio-wave, and a sample is ionized by being put in the generated plasma. Then, the ionized sample is introduced into a vacuum through a pinhole, and is analyzed. The ICP-MS is disclosed, for example, in xe2x80x9cBUNSEKIxe2x80x9d, Journal of The Japan Society for Analytical Chemistry, p 342, vol. 5, 1996.
In the ICP-MS, since argon gas is used to generate plasma, a large quantity of matrix ions, such as Ar+ ions, ArO+ ions, etc., which are produced from argon gas used, is generated. Therefore, there has been a problem in that ions such as Ca+ ions, Fe+ ions, etc., whose mass is near that of the matrix ion, can just barely be detected. Thus, a microwave induced plasma-mass spectrometer (hereafter abbreviated to MIP-MS), in which a microwave is used to generate plasma, has been developed. In the MIP-PS, since the high energy density can be obtained by concentrating energy into a narrow space, nitrogen or helium gas can be used as plasma source gas, which in turn can prevent the generation of argon gas related matrix ions. Consequently, it has become possible to analyze elements such as calcium, iron, and so on, with a high sensitivity. The conventional technique of the MIP-PS is disclosed, for example, in Japanese Patent Application Laid-Open Hei 1-309300. Meanwhile, it is known that since the plasma temperature in the MIP-MS is lower than that in the ICP-MS, the analysis sensitivity of the ICP-MS is higher for analyzing elements with high ionization potential than that of the MIP-MS.
The plasma-mass spectrometer such as the ICP-MS, the MIP-MS, etc., is generally used in the field of the analysis for environmental evaluation.
Meanwhile, these spectrometers are equipped with a protection cover to prevent an electrical shock, a heat burn, an influence of a high-frequency radio-wave on a human body, etc.
Here, because particles such as neutral particles, which pass through the mass spectrometer, may reach a detector, and may become detection noises, the mass spectrometer is sometimes composed so that the detector is located in a direction, altered from that in which ions are ejected from the ion ejection hole of the mass spectrometer. For the purpose, a deflecting device is located between the mass spectrometer and the detector. Such a deflecting device is disclosed in Japanese Patent Application Laid-Open Hei 9-161719 and Japanese Patent Application Laid-Open Hei 9-190797. Since the generated neutral particles which become noise sources go straight, they cannot reach the detector. On the other hand, the selected and ejected ions are led by the deflecting device to the detector, and are so detected.
In the plasma ion source-mass spectrometer such as the ICP-MS, the MIP-MS, etc., although various types of mass spectrometers can be used, the use of a quadrupole ion-trap type (referred to simply as ion-trap type) mass spectrometer has recently been tried. It is known that argon gas related molecular ions (ArO+, Arcl+, etc.), or metal oxide ions (CaO+, etc.), which hinder the analysis, can be decomposed by collisions among them in the quadrupole ion-trap type mass spectrometer.
As mentioned above, it has become possible to provide the plasma ion source-mass spectrometer with a new function which can decompose molecular ions hindering the analysis, constructed by combining the plasma ion sources such as the ICP, the MIP, etc., and an ion-trap type mass spectrometer. However, in the ion-trap type mass spectrometer, since the plasma-confinement potential is distorted by the space-charge effect if a large quantity of ions is confined in the mass spectrometer, this may deteriorate the fundamental analysis performances such as the mass resolution. Also, in the plasma ion source-mass spectrometer, since there is a large quantity of plasma gas related ions, and they are confined in the mass spectrometer, the induced space-charge effect becomes great.
An objective of the present invention is to provide a mass spectrometer which can prevent the degradation of its fundamental analysis performances, such as mass resolution, by reducing the above space-charge effect, so as to suppress the distortion of the confinement potential due to this space-charge effect.
Further, another objective of the present invention is to realize a lasting stable analysis in a plasma ion source-mass spectrometer which uses an ion-trap type mass spectrometry unit, by protecting an ion detector from being over-exposed by particles.
The using of a comparatively large size mass spectrometry unit has been investigated for a plasma ion source-mass spectrometer which uses an ion-trap type mass spectrometry unit. The size increase of a mass spectrometer increases the volume of the mass spectrometer, which in turn will reduce the space-charge effect of plasma ions. On the other hand, if the size of the mass spectrometer is increased, this causes a disadvantage in that the measurable upper limit of mass number decreases. However, since the required upper limit of mass number is approximately 250 in the element analysis field in which the plasma ion source-mass spectrometer is mainly used, the size of a mass spectrometer can be increased within a range satisfying the above condition.
Thus, a plasma ion source-mass spectrometer which uses an ion-trap type mass spectrometry unit with a size larger than that of conventional type of plasma ion source-mass spectrometry units has been developed and evaluated. That is, the smallest inside diameter r0 in the ring electrode of the developed mass spectrometry unit is 16 mm which is larger than 7 mm or 10 mm of the conventional type of plasma ion source-mass spectrometry units. From the evaluation results, it has been found that the developed mass spectrometry unit has a new subject.
In the ion-trap type mass spectrometry unit, a mass spectrum is obtained by alternately setting an ion-confining (or trapping ) period and a mass-analyzing period. An electrode called an ion-stopping electrode is usually located between a mass spectrometry unit and a detector. During the ion-stopping period, positive voltage (typically +300V) is applied to the ion-stopping electrode so that ions do not reach the detector. On the other hand, during the mass-analyzing period, negative voltage (typically xe2x88x92300V) is applied to the ion-stopping electrode so that ions can pass through this electrode and reach the detector. In this way, by switching the sign of the voltage applied to the ion-stopping electrode, the timing of when ions reach the detector is controlled.
However, it was found that the detector actually outputs a high-level signal even during the ion-stopping period. Thus, it was proved that there is a problem in which the detector may break down due to the influence of an over current, or, the lifetime of the detector may be remarkably decreased.
It should be noted that, the efficiency of confining plasma gas related ions is low under the conditions suitable for confining ions of a sample. Therefore, one cause of the above problem is that many of the plasma gas related ions, which have reached the mass spectrometry unit, possibly pass through it and reach the detector. Accordingly, since a large quantity of plasma gas related ions pass through the mass spectrometry unit and reach the detector despite the preventing of ion-ejection from the mass spectrometry unit by applying positive voltage to the ion-stopping electrode, a load on the detector may become excessive in the state in which a high-level amplification-voltage is applied to the detector so as to detect ions of an infinitesimal sample.
Although the energy of ions which enter the mass spectrometry unit is adjusted to be several eV, the energy of the ions in the mass spectrometry unit may be increased sometimes to more than 300 eV due to the acceleration which the ions receive in the mass spectrometry unit. Also, a high-frequency radio-wave with an amplitude of xc2x17 kV is generally applied to a ring electrode of the mass spectrometry unit. Further, since the potential of an endcap electrode in the mass spectrometry unit is maintained to be 0V, the potential near the center of the space surrounded by the endcap electrodes 72, the ring electrode 71, and the ion-stopping electrode 73, in the mass spectrometry unit does not increase to a high voltage such as that applied to the ring electrode. However, it is considered that if a comparatively large size mass spectrometry unit is used, the distance between the ring electrode and the endcap electrode is increased, which in turn makes the variation of the potential near the center of the mass spectrometry unit large, and the acceleration of the ions in the mass spectrometry unit becomes great.
By increasing the voltage applied to the ion-stopping electrode, it may be possible to prevent the ions, which are accelerated to more than 300 eV in the mass spectrometry unit during the ion-confining period, being ejected, and reaching the detector. However, it was proved that this method cannot completely resolve the above problem. Although the signal level of the detector decreased to about one tenth when the voltage of +800V was applied to the ion-stopping electrode during the ion-confining period, a continuous signal of a level about 100 times higher than that of a signal observed during the mass-analyzing period, was observed in the above situation. Further, even if voltage higher than +800V was applied to the ion-stopping element, the level of the detected signal was not remarkably changed. From the above observation, some neutral particles may relate to the high-level signal of the detector during the ion-confining period. For example, it is possible that ions accelerated in the mass spectrometry unit cause charge-exchange reactions with neutral gas molecules, reach the detector as neutral particles with high energy, and are detected.
The present invention resolves the above problem by preventing the undesired ions which have passed the aperture of the ion-stopping electrode from reaching the detector during the ion-confining period.
Thus, to achieve the above objectives, the present invention provides a mass spectrometer comprising: an ion source-generating unit to ionize a sample; an ion-trap type mass spectrometry unit for analyzing mass of respective ions introduced from the ion source-generating unit; a detector to detect ions ejected from the ion-trap type mass spectrometry unit; and a switching device for switching the voltage applied to the detector from a first voltage value to a second voltage value just before analyzing mass of ions is started in the ion-trap type mass spectrometry unit.
In the above mass spectrometer, wherein the detector is located at a position in a direction different from that in which ions are ejected from an outlet of the ion-trap type mass spectrometry unit.
Further, the present invention provides a mass spectrometer comprising: an ion source-generating unit to ionize a sample: an ion-trap type mass spectrometry unit for analyzing the mass of respective ions introduced from the ion source-generating unit; a detector to detect ions ejected from the ion-trap type mass spectrometry unit; and an ion-deflecting device situated between the detector and the ion-trap type mass spectrometry unit.
In the above mass spectrometer, the ion-deflecting device is composed of a plurality of electrodes, and at least one of the electrodes is grounded.
Also, in the above mass spectrometer, the ion-deflecting device is composed of a first electrode and a second electrode, and the first electrode is connected to power sources for applying the positive and negative voltage, respectively.
Furthermore, a mass spectrometer comprising: an ion source-generating unit to ionize a sample; a first ion-deflecting device for deflecting ions generated in the ion source-generating unit; a ring electrode with a more than 16 mm diameter at the position where high-frequency voltage is applied to the deflected and introduced ions in order to analyze mass of the respective ions; first and second dish electrodes, in each of which an aperture is shaped, arranged at both sides of the ring electrode, respectively; a detector to detect ions ejected from the aperture of the second dish electrode; and a second ion-deflecting device, which is located between the second dish electrode and the detector, for deflecting the ions ejected from the aperture of the second dish electrode.